This invention relates to circuits and methods for discharging DC filter capacitors in electric power systems.
Certain electric power systems, such as those used for aircraft applications, include an electric generator having an output which is rectified to produce a DC voltage on a pair of conductors. This DC voltage may be transmitted directly to a load or it may be used by an inverter to produce a constant frequency AC output voltage. In both cases, typical voltage regulators used in the system sense the output voltage, compare it to a reference, and amplify a resulting error signal to produce exciter field current in the generator, thereby controlling the output of the system. A filter capacitor is connected between the DC conductors to filter the DC voltage. This capacitor must be sufficiently large to provide a low impedance source for an inverter in a DC link variable speed constant frequency system, or to filter the ripple in a DC system. Typical capacitor values are 55 microfarads for 270 Vdc systems or 165 microfarads for 115 Vac, 20 kW, variable speed constant frequency systems. In both cases, the DC voltage at the capacitor is about 270 Vdc under normal operating conditions.
At rated load, the effects of the filter capacitor are insignificant, amounting to only a fraction of a millisecond of energy storage. At light loads, however, the discharge time may become significant. In extreme cases, for a DC system, a 100% load removal will cause a large overvoltage on the filter capacitor, which has no discharge path except the leakage of the filter itself.
For an AC system, losses in the inverter provide a minimum load to the filter, typically discharging the overvoltage in about 10 milliseconds. DC systems, however, require a preload to produce the same results. The continuous losses of a preload result in an unsatisfactory reduction in system efficiency.
With recent improvements in voltage regulator control circuits, the filter capacitor discharge may be the limiting factor in transient response improvement. In a DC link variable speed constant frequency system, a rated load removal causes a temporary increase in output voltage until the control loop returns the output voltage to its normal level. Attempts to improve the response of the system by increasing the gain of the control loop may result in an underdamped system with an undershoot nearly as large as the overshoot voltage. This non-linearity in transient response is caused by the slow discharge time of the DC link filter capacitor. If the generator voltage drops faster than the link voltage, the voltage control loop is broken. Generator voltage is forced down until the link capacitor discharges to rated voltage. Generator excitation then increases but the system voltage continues to sag until the generator begins to recharge the DC link capacitor. With the control loop again closed, the output increases back to rated voltage with very little overshoot if the control loop is well damped.
With the rate of capacitor discharge being the limiting factor in transient response, it is desirable to provide a circuit for discharging the DC filter capacitor when the power system experiences a sudden decrease in load or load removal.